Techniques using an array of a plurality of optical fibers as a light source for a light scanning unit have been disclosed (see JP-A-2004-258566 and JP-A-2001-66438). The alignment accuracy of core portions is an essential problem in light scanning with high accuracy.
When an array of a plurality of optical fibers is used as a light source for a light scanning unit, it is generally a problem that the diameter of a core portion of each optical fiber is significantly smaller than the diameter of a clad portion thereof. FIGS. 13 and 14 show the relationship between an aperture diameter and a beam spot diameter.
In FIGS. 13 and 14, the reference numeral 11 represents a core portion; 12, a clad portion; 91, a light source portion; 92, a to-be-scanned surface; 93, a lens system; 94 and 95, beam spots; and 96, an aperture member. The light source portion 91 and the to-be-scanned surface 92 are generally disposed in an optically conjugate relationship by the lens 93 for the reason which will be described later.
Assume that the clad portions 12 are disposed contiguously in the light source portion 91. In this case, the ratio of the diameter of each clad portion 12 and the diameter of each core portion 11 is equal to the ratio of the scanning line interval to the diameter of each beam spot 94, 95 in the to-be-scanned surface 92. When the diameter of the core portion 11 is significantly smaller than the diameter of the clad portion 12, the diameter of the beam spot 94 becomes significantly smaller than the scanning line interval. Thus, as shown in FIG. 13, contiguous scanning lines cannot be formed.
A solution to this is to use a method in which the diameter of a beam emitted from each core portion 11 is narrowed by the aperture member 96 so as to increase the diameter of the core portion 11 equivalently as shown in FIG. 14. However, the beam is partially blocked by the aperture member 96. There is a problem that the luminous energy of the beam is lost.
As a solution to this problem, there has been proposed a method in which a predetermined thickness of an outside circumferential portion of each clad portion is removed by etching or the like so as to reduce the outer diameter of the clad portion (see JP-A-2000-147296). Thus, it is possible to reduce the intervals of the core portions of the optical fibers.
However, there is a problem that etching easily leads to a variation in thickness of removal. There is also a problem that the workability is low when the clad portions whose diameters have been narrowed by etching are fixed to groove portions of a substrate individually by a method such as bonding.
In order to miniaturize light guide parts, there is another proposal where optical fibers are fixed to two substrates respectively, and the substrates are put on top of each other so as to arrange the optical fibers in two stages (see JP-A-2001-83361 or JP-A-2004-122400). According to this method, the light guide parts can be miniaturized as compared with the case where optical fibers are arranged on a single substrate.
However, in the case where the light guide parts are used as light sources for a light scanning unit, there is a problem that the light sources arrayed in two stages lead to separation when beams emitted from the light sources are incident on a light deflecting unit such as a polygon mirror. As a solution to this problem, there is a method where the misalignment is corrected by use of a lens or the like (see JP-A-56-69611). However, due to a complicated configuration, there arises a problem that the optical path length is increased, or the like.
When the light source portion and the to-be-scanned surface are not disposed in an optically conjugate relationship, the ratio of the beam spot diameter to the scanning line interval in the to-be-scanned surface becomes larger than the ratio of the diameter of the core portion to the diameter of the clad portion in the light source portion. Therefore, there are conditions where contiguous scanning lines can be formed. However, this means the state where beams are not focused on the to-be-scanned surface. The beam diameters become unstable spatially. Thus, this method is not practical.